Add `unlayerAll` function to kernel
Fix typo in `follow` and remove layer reference
Reference screen directly in `mouseDown`
Change layer system

Now layers don't talk to each other and are run separately. Every tagged
item is tagged per layer. During stepping loop the `activeLayer`
attribute is changed to reflect the current layer. This automatic
separation has the side effect of requiring explicit reference to other
layers through the `tagged` function on them.